Incidentally you can check the text for years back to at least 1995 with this URL and change the year -
https://www.govinfo.gov/content/pkg/USCODE-1995-title23/html/USCODE-1995-title23.htmIt appears federal $ can be used in constructing NEW toll roads that can be assigned interstate status based on the change I pointed out.
The FAQ Froggie cited clearly says new construction.
(a)(1)(F) seems to say federal $ cannot be used to reconstruct an interstate to then make it a toll road.
Designating an interstate is actually covered in 23 USC 103(c)(4) which does not say toll roads are ineligible. The 1995 version of 23 USC 103 has less language (and is actually 103(e)). The 103(c)(4) language has been the same since 1998.
My extensive experience with Nuclear Regulatory Commission regulations is that if there is no explicit language prohibiting something, you can in theory do it. But there could be FWHA policy that says they won't approve an existing toll road to become an interstate. There are regulations that are in place to implement 23 USC 103.
23 CFR 470.111 governs designating interstates. It references Appendix A and B of 23 CFR 470 (scroll down a short distance from 470.111) for criteria. Toll roads are again not mentioned.
Drilling down further to see if FHWA policy might still preclude designating an existing toll road an interstate, I found this guidance which goes into construction-related activities including the mainstreaming of the pilot programs -
https://www.fhwa.dot.gov/map21/guidance/guidetoll.cfm. But I didn't find anything related to designating existing toll roads. Which I conclude has always been legal, since I-240 OKC was approved; I-44's extension southwest of OKC years ago and I-335 Kansas years ago were to long-existing toll-roads.